1. Field of the Invention
The present invention relates generally to sensors, and more particularly, to accelerometers and inertia based gyros that are hardened to multi-directional shock experienced during high-G (G indicating the gravitational acceleration of around 9.8 m/sec2) firing setback and set-forward.
2. Prior Art
The state of art in shock resistant accelerometer and inertia based gyro design is to reduce the size of the moving proof mass (gyroscopic proof mass for the case of inertia based gyros), thereby reducing the related forces, moments, and torques that are generated in the presence of high acceleration levels, i.e., when the accelerometer and gyro experiences shock or impact loading. Hereinafter and for the sake of simplicity and since the disclosed locking mechanisms apply equally to both accelerometers and inertia based gyros of various type, all such sensors are referred to as accelerometers. In general stops are also provided in the path of the moving component(s) of the accelerometer to limit its maximum deflection to protect such components from failure. The introduction of MEMS technology in recent years has made it possible to reduce the size of the proof mass significantly, independent of the accelerometer type and its mechanism of operation. All existing accelerometer designs, however, generally suffer from the following operational and/or performance deficiencies.
The most important shortcoming results from the reduction of the size of the proof mass since the sensitivity of an accelerometer is directly related to the relative size of its proof mass, even if the shape and design of the accelerometer structure is optimally selected. As a result, since highly accurate accelerometers are required for smart munitions guidance and control during their flight (sometimes resolutions in 1/100 or even 1/1000 of one G) and other similar applications, an accelerometer that can withstand tens of thousands of one G with a floating proof mass cannot be designed to provide such levels of precision.
Another major shortcoming is related to the significant amount of settling time required for the accelerometer to settle within an acceptable level following shock loading. Many types of sensors, particularly accelerometers, rely upon the deflection of one or more elastic structural elements of the sensor to make their sensory measurements. When subjected to firing setback or set-forward firing shock, which for a sensitive accelerometer or when the firing acceleration is high results in the proof mass to reach its travel limit at its (usually hard) stops, and generally impacting the stops. The sensor is thereby “saturated” and the mechanical energy stored in the sensor components in the form of potential energy in the elastic elements and kinetic energy in the proof mass and other elements of the sensor will cause the sensor structure to begin to vibrate following such impact events. The time until the vibration ceases or reduces to an acceptable value is referred to as a settling time. The settling time is particularly important for accelerometers used in guns or similarly fired projectiles and that are intended to be used for navigation and/or guidance and/or control purposes.
It is noted that accelerometers that are designed without proof mass travel limit stops and that can provide high sensitivity of the aforementioned order and that can tolerate high G shocks of the order of tens of thousands without permanent damage or change in their characteristics are yet to be conceived. This statement is also true for accelerometers with proof mass travel limit stops when subjected to high G shocks of over 30,000-50,000 Gs. This is the case since due to the nature of all proof mass based accelerometers, high sensitivity to low acceleration levels make them highly susceptible to shock loading damage since they rely on relatively large deformations to be induced in the accelerometer mechanism due to small input accelerations.
To alleviate the aforementioned shortcomings of proof mass based accelerometers and other similar inertia based sensors, active and passive mechanisms are disclosed in U.S. Pat. No. 6,626,040 that are used to lock the proof mass (and potentially other moving elements of the sensor) to the base structure of the sensor, preferably at its null position or near its (currently experienced) acceleration level, when the accelerometer is subjected to a shock with acceleration levels above a certain predetermined threshold. As a result, the proof mass and other moving elements of the sensor are protected from impacting their stops (or other elements of the sensor or its packaging if no strops are provided) and damaging the proof mass and/or other elements of the sensor. In addition, the generated dynamic forces acting on the proof mass and other elements of the sensor can better be distributed and supported.
In the above patent, the inventors disclose different embodiments for providing locking mechanisms for proof mass and other moving elements of inertia based sensors (hereinafter, all such mechanisms are referred to as simply “locking mechanisms”), particularly for accelerometers. These embodiments may be divided into the following two basic classes of locking mechanisms for proof mass and other moving elements of such inertia based sensors:                1—Active type of locking mechanisms: In this class of locking mechanisms, the means of actuating the locking elements is an active element such as an element that is powered electrically to generate a mechanical displacement and/or rotation.        2—Passive type of locking mechanisms: In this class of locking mechanisms, the means of actuating the locking elements is the dynamic force and/or torque and/or bending moment that is generated by the acceleration experienced by the sensor when the acceleration level (for example due to shock loading) reaches a predetermined level.        
It is noted that in all the disclosed embodiments of the U.S. Pat. No. 6,626,040 the locking action is achieved by providing mechanical elements that would constrain the motion of one moving element relative to another moving or fixed (generally meant to mean the structure of the sensor) element.
The aforementioned class of active type of locking mechanisms, including those embodiments that are disclosed in the U.S. Pat. No. 6,626,040, has certain advantages over the aforementioned class of passive type of locking mechanisms. They class of active type of locking mechanisms, however, suffer from shortcomings that make them unsuitable for a large number of applications, including those of guided gun-fired munitions, mortars, rockets and the like. The main advantages of the class of active type of locking mechanisms include the following:                1—The locking action may be initiated based on any sensory stimuli and since certain electronics circuitry, logic and/or processing unit must be provided, a wide range of choices, including the use of certain algorithms becomes possible for initiating the locking action. In fact, the locking action may be initiated even before certain event occurs or is timed to occur. As a result, this class of locking mechanisms provides a high level of flexibility to the user.        2—When using the locking mechanism to protect the proof mass and other moving elements of an inertia-based sensor (device) from shock loading, this class of locking mechanisms can provide the means to lock the proof mass and other moving elements of the sensor (device) irrespective of the direction of the shock loading. For example, when a round is fired by a gun, it is first subjected to firing (setback) acceleration inside the barrel and then to an opposite set-forward acceleration, which even though is usually a fraction of the setback acceleration (usually around 5-10 percent of the setback acceleration), but is still significantly higher than a desired threshold for locking the proof mass and other moving elements of a sensor to protection against damaged. The use of active locking mechanisms in sensors such as accelerometers used in gun-fired munitions, mortars and the like provides the means to lock the proof mass and other moving elements of the sensor during both setback and set-forward acceleration events.        
The main shortcomings of the class of active type of locking mechanisms, including the shortcomings that make then unsuitable for most gun-fired munitions, mortars, rockets and the like, include the following:                1—Active locking mechanisms require event detection components such as sensors to detect the predetermined events, such a shock induced acceleration threshold, to trigger the actuation of the locking mechanism.        2—Active locking mechanisms require onboard electronics and/or logics circuitry and/or processing units for event detection to initiate the locking action or for timing such locking action initiation and to perform other related decision making activities.        3—Active locking mechanisms require actuation devices to operate. Such actuation devices are usually powered electrically, and may be designed to operate using the principles of electrical motors or solenoids, or active materials such as piezoelectric materials based elements.        4—In addition to requiring the aforementioned components to operate, active locking mechanisms also require electrical energy to power these devices. This requires the device using a sensor equipped with such active locking mechanism to be powered before an event that requires locking mechanism activation could occur. For munitions and other similar applications, this requirement translates to a need for onboard power sources to power sensor before launch. In addition, the total amount of power that required for the operation of the sensor becomes significantly higher than sensors equipped with passive locking mechanisms. The said requirement of electrical power availability prior to firing and/or the significantly higher power requirement as compared to sensors equipped with passive locking mechanisms make sensors equipped with active locking mechanisms undesirable for gun-fired munitions, mortars and the like applications.        5—In addition, devices using sensors equipped with locking mechanisms, particularly munitions, must also tolerate shock loading due to accidental events such as, for example, drops from up to 7 feet over concrete (hard) surfaces that can result in impact induced deceleration levels of up to 2,000 G. This means that devices using sensors equipped with active locking mechanisms cannot rely on their locking mechanisms to protect the proof mass and other moving elements of the sensor against such accidental drops since munitions cannot be powered at all times, even during assembly, transportation and storage. This in turn means that such sensors have to be provided with smaller proof mass to allow then to survive such accidental drops, i.e., their sensitivity has to be limited to prevent being damaged during such accidental drops.        
The embodiments of the class of passive type of locking mechanisms disclosed in the U.S. Pat. No. 6,626,040, however, do not suffer from the above shortcomings of the class of active type of locking mechanisms, including the embodiments disclosed in the said patent. The said embodiments of class of passive type of locking mechanisms, however, suffer from the following shortcomings that make them undesirable for a large number of applications, including those of guided gun-fired munitions, mortars, rockets and the like:                1—For gun-fired munitions, mortars and the like, the embodiments of the class of passive type of locking mechanisms disclosed in the U.S. Pat. No. 6,626,040 provide protection to the proof mass and other moving components of the sensor against shock loading generated by the firing (setback) acceleration only and not against the set-forward acceleration which is in the opposite direction to the setback acceleration.        2—Similarly, in case of accidental drops, the proof mass and other moving components of the sensors are protected only if the device impacts a hard surface in the direction causing sensor acceleration in the direction of the firing setback acceleration. Otherwise if the impact occurs on the opposite side of the device, i.e., if the impact induced acceleration of the sensor is in the direction of the set-forward acceleration, then the proof mass and other moving components of the sensors are no longer protected against the impact induced shock.        
The aforementioned class of passive locking mechanisms taught in the U.S. Pat. No. 6,626,040 and its aforementioned shortcomings are best described the embodiment of FIGS. 1a and 1b of the said patent. Referring now to FIGS. 1a and 1b, there is an accelerometer 100 shown schematically therein, which is intended to measure acceleration a in the direction 101. The accelerometer consists of a proof mass 102 which is rigidly attached to a relatively rigid base 106 (plate), a cantilever (bending) type of elastic element 103 with an equivalent spring rate k at the location of the proof mass 102 and in the direction of the acceleration 101. The proof mass 102 (with mass m) is located a distance 104 (with length l) from the base 105 to which the elastic beam element 103 is rigidly attached. In most MEMS types of accelerometers, the displacing plate 106 forms one side of a capacitor while the other capacitor plate (not shown) is rigidly attached to the base 105. This capacitor will then form the sensor that measures the elastic displacement of the proof mass due to the acceleration in the direction 101.
The basic proof mass locking mechanism of this embodiment consists of locking a first locking mass 108 which is attached to the base 105 by spring 107 on one side and locking a second locking mass 109 and spring 110 on the opposite side of the proof mass base plate 106. The second locking mass 109 is attached to a lever arm 111, which is hinged to the base 105 by the rotational joint 113. The spring 110 is attached to the base 105 on one end and to the lever arm 111 on the other. Opposite to the second locking mass 109 is positioned a moment mass 112 which provides a moment about the hinge joint 113 when the sensor is accelerated in the direction of the arrow 101. The moment mass 112 has a greater mass than that provided by the first locking mass 109, thereby it tends to move the first locking mass 109 upwards due to the acceleration in the direction 101.
The spring rates of the springs 107 and 110 are selected such that at the desired acceleration levels the gap between the first and second locking masses 109 and 108 and the plate 106 begin to close. A spaced locking stop 114 is located along the plate 106 to lock the plate 106 at the level dictated by the position of the locking stop 114. As a result, when the acceleration in the direction 101 reaches the selected level, the first and second locking masses 109 and 108 close the aforementioned gap, and thereby hold the base 106 and the proof mass 102 stationary at its null point, as shown in FIG. 1b. 
In general, the springs 107 and 110 are preferably preloaded, i.e., provide a preset force in the direction of providing the required gap between themselves and the plate 106, and as the acceleration level reaches the desired maximum level, they will begin to close the gap. A basic mechanism to lock the proof mass 102 and/or other moving components of an accelerometer is described above using an elastic beam type of accelerometer. The design, however, can be seen to be applicable to almost all accelerometer and inertia based gyro designs, particularly those constructed using MEMS technology, such as those employing a linear displacement, a ring type, and a torsional type of accelerometers or gyros.
The basic proof mass locking mechanism taught in the U.S. Pat. No. 6,626,040, FIGS. 1a and 1b, is thereby seen to be capable of locking the proof mass to the base structure of the sensor when the shock acceleration experienced by the sensor is in the direction of the arrow 101 but not in its opposite direction. For example, if the sensor is used in gun-fired munitions, the locking mechanism can be designed to protect the sensor from the firing setback acceleration, but the generally significant set-forward acceleration of the said munitions experienced as the projectile exits the gun barrel can still damage the sensor. The sensor may similarly experience impact induced shock accelerations from two opposite directions similar to setback and set-forwards accelerations due to accidental drops.